1. Field of the Invention
The present invention relates to communication satellite systems. Particularly, this invention relates to systems and methods for bandwidth and power efficient techniques to enhance data throughput for communication satellite systems.
2. Description of the Related Art
Increasing data throughput for a given allocated bandwidth of a communication satellite translates directly into benefits for the satellite operator. These benefits can enhance end-to-end system capabilities for the government or civil customer for example, or increased revenue and market opportunities for a commercial service provider. Communication satellite systems have limited bandwidth and spacecraft power and both government and commercial operators need to maximize the system utility. Thus, there is a need for satellite systems that operate with enhanced data throughput.
Power and bandwidth are two precious commodities of a communication satellite system. Current satellite systems typically achieve approximately 1 to 1.5 bits-per-second-per-hertz (bps/Hz) bandwidth efficiencies using QPSK modulation; for systems with slightly more power to overcome the denser signaling format, one may achieve approximately 2 bps/Hz using 8PSK modulation. Because non-band-limited QPSK and 8PSK have equal power in each possible symbol, they are considered xe2x80x9cconstant-envelopexe2x80x9d modulation schemes. This characteristic reduces the waveform""s vulnerability to the nonlinear distortions inherent to power efficient, space-borne transmitters. Although band-limiting (also known as pulse shaping) eliminates the strict constant envelope characteristic, the band-limited waveforms remain quite robust to nonlinear distortions.
Transmission with higher bandwidth efficiency enables more data to be transmitted for a given spectral bandwidth. However, the penalty of constant envelope waveforms becomes untenable. Moreover, the more densely packed bandwidth-efficient constellations are more vulnerable to distortion effects. The impact is particularly significant for satellite links where nonlinear amplifiers are used to maximize the efficient utilization of on-board power. A prosaic approach might be to operate the spacecraft traveling wave tube amplifier (TWTA) in a backed off mode, several dB from its saturation point, and achieve more linear performance. However, this approach wastes spacecraft power, since the amplifier efficiency degrades significantly as it is backed off from saturation.
This invention describes a technique and implementation that intentionally corrupts (xe2x80x9cpredistortsxe2x80x9d) the high power amplifier input to create the desired signal at the amplifier output. Related digital modulators which utilize random access memory (RAM) based pre-distortion have been applied to similar link architectures. In this scheme, several consecutive symbols are used to address the RAM. The RAM contents contain digital representation of the desired predistorted waveform. However, while highly flexible, the RAM size for these other techniques becomes prohibitive for higher order modulations with band limiting or significant dispersive distortion. For example, analyses indicate that achieving 5 bits per second per Hertz of available bandwidth requires 64-ary modulation and a pre-distortion that considers 32 consecutive symbols. The device would need to provide two I and Q samples per symbol each with at least 6 bits of resolution. Thus the RAM would have 193 address bits (6 bits/symbolxc3x9732 symbols+1 bit for even/odd sample) and a width of 12 bits (I and Q at 6 bits each). Consequently, the necessary RAM would be 1.9xc3x971058 bytes. (To illustrate the infeasibility of the RAM based approach to such higher order modulations, this RAM would require the mass of approximately 3,500 suns using a hypothetical storage device requiring only a single silicon atom per bit of storage.)
Another approach for mitigating nonlinear distortions is to use on board radio frequency (RF) devices called linearizers to counteract the high power amplifier (HPA) nonlinearity. The cascade of a carefully tuned linearizer with the HPA creates the desired linear amplifier response. This analog approach, however, requires significant hardware on board the spacecraft (increasing size, weight and power consumption of the payload). In addition, it is extremely time-consuming and difficult to tune the extra hardware properly over temperature variations and other lifetime equipment variabilities. Consequently, this approach may not provide sufficient linearity to enable multi-amplitude signal transmission. Furthermore, frequency dependent variations in the HPA, linearizer, and associated RF components make this approach even more difficult for very wideband channels.
There is a need for systems and methods for improving the data throughput or bandwidth efficiency for satellite transmission systems without the aforementioned difficulties. The present invention meets these needs.
The present invention enables transmission of bandwidth efficient modulation formats capable of achieving approximately two to five times the per channel data throughput of current satellite systems with practical spacecraft power levels and hardware implementations.
Embodiments of the invention employ a digitally-implemented, self tuning technique to mitigate both linear and nonlinear distortions typical of a wideband satellite channel utilizing an HPA. The invention is applicable to very wideband channels (e.g.,  greater than 1 GHz), distortion limited, direct satellite downlinks, as well as typical commercial, civil, and tactical transponded satellite channels. Further, the invention enables extremely power-efficient multi-amplitude signaling through a nonlinear HPA.
In contrast to the RAM-based digital modulators discussed above, the present approach utilizes algorithmic pre-distortion to-sidestep the RAM size by computing each value when it is needed rather than storing all possible required values. The present algorithmic pre-distortion architecture is motivated by the physical limitation of the related approach and has all flexibility required to mitigate distortions that occur in practice.
The invention provides channel distortion mitigation to enable a two-fold (or greater) increase in per-channel satellite capacity over the current state-of-the-art without sacrificing enormous amounts of spacecraft power and without requiring heroic analog HPA linearization efforts. This technology can enable information superiority and provide a key element of emerging system architectures. In commercial programs, the improved bandwidth efficiency enables higher throughputs and thus increased revenues. Moreover, the power efficient, bandwidth efficiency of the present invention allows conventional transponders and low cost terminals to achieve key threshold rates such as that of optical carrier standard, OC-3 (155 Mbps). In one notable exemplary application, the invention can be applied to a digital cinema program distribution cinema to enable efficient delivery of the high quality digital media via satellite, enabling current satellites and inexpensive receive terminals to complement the high capacity distribution afforded by terrestrial fiber-optic service.
A typical embodiment of the invention includes a predistortion processor for predistorting a digital data source signal to reduce transmission distortion and an adaptive algorithm processor for controlling the predistortion processor according to a received a feedback signal derived from the transmitted signal. The feedback signal can be derived at the transmitter output or via feedback messages from a receive location. In the former case, a common local oscillator can be used to provide a timing signal to both the modulator and demodulator. In the latter case, the feedback signal may be communicated to the adaptive algorithm processor via a low data rate return path, either through a satellite return link or via an alternative path such as a telephone line MODEM.
The predistortion processor can include linear and non-linear processing. The linear processing can employ a zero forcing equalization (ZFE) algorithm implemented with an asymmetric finite impulse response digital filter (e.g., four independent real finite impulse response functions). Coefficients of the digital filter can be controlled by adaptive algorithm processor. The non-linear processing can include a complex gain multiplying the digital data source signal where the complex gain depends upon a monotonic function (e.g., power or voltage) of the transmitted data source signal. A lookup table can be used to quickly determine the proper complex gain. The lookup table (values) can be adjusted by the adaptive algorithm processor using a gradient technique.
A pulse shaping digital filter (e.g., a symmetric finite impulse response filter) can also be applied to the digital data source signal along with the predistortion processing. The filter can comprise two substantially identical real finite impulse response functions to achieve the desired degree of bandlimiting.